7545 datasheet(56/62 Pages) RENESAS

56 / 62 page

Rev.1.06

Mar 07, 2008

Page 56 of 59

REJ03B0140-0106

7545 Group

5. Read-modify-write instruction

Do not execute a read-modify-write instruction to the read

invalid address (SFR).

The read-modify-write instruction operates in the following

sequence: read one-byte of data from memory, modify the data,

write the data back to original memory. The following

instructions are classified as the read-modify-write instructions

in the 740 Family.

(1) Bit management instructions: CLB, SEB

(2) Shift and rotate instructions: ASL, LSR, ROL, ROR, RRF

(3) Add and subtract instructions: DEC, INC

(4) Logical operation instructions (1’s complement): COM

Add and subtract/logical operation instructions (ADC, SBC,

AND, EOR, and ORA) when T flag = “1” operate in the way as

the read-modify-write instruction. Do not execute the read

invalid SFR.

When the read-modify-write instruction is executed to read

invalid SFR, the instru ction may cause the following

consequence: the instruction reads unspecified data from the area

due to the read invalid condition. Then the instruction modifies

this unspecified data and writes the data to the area. The result

will be random data written to the area or some unexpected

event.

NOTES ON PERIPHERAL FUNCTIONS

Notes on I/O Ports

1. Pull-up control register

When using each port which built in pull-up resistor as an output

port, the pull-up control bit of corresponding port becomes

invalid, and pull-up resistor is not connected.

Pull-up control is effective only when each direction register is

set to the input mode.

2. Notes in stand-by state

In stand-by state*1 for low-power dissipation, do not make input

levels of an input port and an I/O port “undefined”.

Pull-up (connect the port to Vcc) or pull-down (connect the port

to Vss) these ports through a resistor.

When determining a resistance value, note the following points:

• External circuit

• Variation of output levels during the ordinary operation

When using a built-in pull-up resistor, note on varied current

values:

• When setting as an input port : Fix its input level

• When setting as an output port : Prevent current from flowing

out to external.

The output transistor becomes the OFF state, which causes the

ports to be the high-impedance state. Note that the level becomes

“undefined” depending on external circuits.

Accordingly, the potential which is input to the input buffer in a

microcomputer is unstable in the state that input levels of an

input port and an I/O port are “undefined”. This may cause

power source current.

*1 stand-by state : the stop mode by executing the STP

instruction

3. Modifying output data with bit managing instruction

When the port latch of an I/O port is modified with the bit

managing instruction*1 , the value of the unspecified bit may be

changed.

I/O ports are set to input or output mode in bit units. Reading

from a port register or writing to it involves the following

operations.

• Port in input mode

Read: Read the pin level.

Write: Write to the port latch.

• Port in output mode

Read: Read the port latch or read the output from the

peripheral function (specifications differ depending on the

port).

Write: Write to the port latch. (The port latch value is output

from the pin.)

Since bit managing instructions*1 are read-modify-write

instructions,*2 using such an instruction on a port register causes

a read and write to be performed simultaneously on the bits other

than the one specified by the instruction.

When an unspecified bit is in input mode, its pin level is read and

that value is written to the port latch. If the previous value of the

port latch differs from the pin level, the port latch value is

changed.

If an unspecified bit is in output mode, the port latch is generally

read. However, for some ports the peripheral function output is

read, and the value is written to the port latch. In this case, if the

previous value of the port latch differs from the peripheral

function output, the port latch value is changed.

*1 Bit managing instructions: SEB and CLB instructions

*2 Read-modify-write instructions: Instructions that read

memory in byte units, modify the value, and then write the

result to the same location in memory in byte units

4. Direction register

The values of the port direction registers cannot be read.

That is, it is impossible to use the LDA instruction, memory

operation instruction when the T flag is “1”, addressing mode

using direction register values as qualifiers, and bit test

instructions such as BBC and BBS.

It is also impossible to use bit operation instructions such as CLB

and SEB and read-modify-write instructions of direction

registers for calculations such as ROR.

For setting direction registers, use the LDM instruction, STA

instruction, etc.

7545 Datasheet(PDF) 56 Page - Renesas Technology Corp

7545 Datasheet(HTML) 56 Page - Renesas Technology Corp

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62

Datasheet Download

Link URL